The utilization of bipolar junction transistors as switching elements in switching power supplies has been limited by their disposition to heat excessively at high switching frequencies. This, in turn, degrades their turn-off characteristics, which leads to additional heating. Recent development of high gain, high current bipolar transistors, such as the 788-769 and complementary 688-695 series bipolar transistors manufactured by Zetex plc, have led to switching regulator implementations at currents of two amperes with switching frequencies in excess of 100 KHz. Still, high currents and high voltages have restricted the choice of pass elements in switching regulators to MOSFETS. This is owing to the beta limitations of bipolar junction transistors at turn-on.
At turn-on, a switching element is compelled to commutate another switching device, typically a freewheeling diode, and reverse the voltage on an inductive element. This action is accompanied by extremely high instantaneous currents. As beta limitations in a bipolar junction transistor make it impossible to transit this state with any rapidity, the result is that the beta limit current is conducted by the transistor at full voltage while the circuit's capacitive elements are commutated. Each turn-on of the switch accumulates heat energy equal to the product of the input voltage, the beta current limit, and the time interval between the onset of switch drive and the actual voltage change at the inductor. Obviously, increasing the number of switch turn-ons per second has the effect of increasing the accumulated heat. The applicability of a device thus becomes limited by the ability to remove this accumulated heat or its ability to maintain its current gain at high currents and thus minimize its transit time through this region of high power dissipation.
MOSFETs overcome this problem by being specified to have an "on" resistance, which under conditions of constant current, would make them competitive with the Vcesat of a bipolar junction transistor ("BJT"). This necessarily makes them substantially larger than the competitive BJT. They are typically capable of ten times their rated continuous current in a pulse mode. They can, therefore, transit the high current region of tun-on extremely rapidly, but that is also their bane in switching applications, as their Miller effect capacitances are hundreds of times larger than those of BJTs. This makes them prone to gate oscillations which can punch through their gates, and renders the drive circuitry much more complex. Their increased size also enables them to reject heat more efficiently.
Therefore, there is a need for a solution to the problem of overheating of BJTs used as switching elements in power supplies at high frequencies. The present invention solves this problem.